The treatment of wastes, particularly livestock waste, is a critical problem in the majority of developed countries, as it causes grave environmental and economic concerns. Large farms—10,000 and more animals—exert pressure on the ecosystem and present problems difficult to solve both in the short-term and the long term. In an attempt to prevent ecological disaster, various countries have legislated limitations to the amount of livestock wastes which may spread per year per surface unit. For example, in Europe a spreading limitation has recently been introduced based on the nitrogen contents of wastes, virtually limiting the size of pig farms.
Traditionally, organic matter containing excessive biological oxygen demand (BOD) has been treated using microbial action. The process begins with the separation of the organic matter from the remainder or liquid fraction, forming a mass of solids in the form of an activated sludge. Treatment typically involves digestion of the organic material through fermentation of the sludge involving aerobic or anaerobic bacterial action or some combination thereof. These processes are used to reduce the chemical oxygen demand (COD) and biological oxygen demand (BOD) of the material to an environmentally safe level. It is also necessary to remove undesirable inorganic materials from the water fraction, which typically contains undesirable quantities of phosphorus and nitrogen compounds including phosphates and nitrates.
Conventional wastewater treatment systems may include pretreatment, primary treatment, secondary treatment, and advanced treatment. Pretreatment includes screening, comminuting (mechanical cleaning of screens by shredding solids to a size which can pass through screen openings), degritting, and grease and scum removal. Primary treatment includes removal of suspended solids from wastewater by clarification and skimming. This typically involves a tank or channel and the steps of reducing flow velocity, settling heavier solids, and skimming relatively light solids. Primary treatment may include anaerobic digestion processes, aerobic digestion processes, or a combination thereof. Primary treatment systems typically include sludge collection mechanisms, sludge suction devices, grit removal devices, and sludge dewatering devices to reduce the volume of sludge to be disposed. Secondary treatment systems are typically aerobic systems including an aeration phase and a clarification phase. Such systems typically include an aeration tank, an air distribution system, a clarifier, sludge collection mechanisms, and sludge removing devices. Advanced treatment includes further removal of suspended and dissolved organic solids by means including filtration and removal of pathogens by oxidation, chlorination or heating, precipitation of minerals, adsorption, or other methods. In additional steps of the advanced treatment, the purified liquor from the clarifier is typically filtered and refined through chlorination, oxidation, or heating.
In the activated sludge process of primary or secondary treatment, microorganisms are contained in an activated sludge and mixed with incoming wastewater, which provides food for the microorganisms whereby more activated sludge is produced. Such mixing is accomplished in an aeration tank or channel. In the aerobic activated sludge process, oxygen is intrinsically mixed with the activated sludge and the wastewater. The microorganisms use the solids as nutrients for their growth and reproduction, converting the suspended organic solids into biomass, carbon dioxide and water. The aerobic activated sludge process therefore typically includes mixing wastewater, activated sludge, and oxygen in an aeration tank; consuming suspended organic solids by bacteria; settling activated sludge in a clarifier; returning the activated sludge to the aeration tank for further treatment; removing purified liquor from the clarifier; and removing and disposing of the final, inert sludge.
Existing processes and installations for the treatment of such livestock wastes, especially for large farms, have generally been large scale operations having high installation costs. There exists a need to provide a compact, low-cost system and method for treating wastewater.